Polyurethanes are synthetic polymers found in widespread commercial and industrial applications. These polymers, characterized by their carbamate (—NH—CO—O—) chemical linkages, are made by methods known in the art, for example by reacting a multifunctional isocyanate with a diol or polyol in the presence of a suitable catalyst. Thermoplastic polyurethanes are characterized by linear polymeric chains that form self-ordering block structures, while thermoset polyurethanes form covalently-bonded crosslinked networks. Through judicious selection of the diisocyanate and diol or polyol constituents, a polyurethane can be engineered to resist degradation from moisture and chemicals while also displaying great flexibility and toughness.
The aforementioned properties make polyurethane coatings and films particularly useful in harsh outdoor environments. In many applications, polyurethane coatings can fulfill decorative purposes while also protecting underlying substrates from environmental weathering, chemical exposure, heat, and/or abrasion. Use of polyurethanes in protective film applications has been described, for example, in U.S. Pat. No. 6,607,831 (Ho et al.) and U.S. Pat. No. 6,383,644 (Fuchs).
Modernly, polyurethane films can be coated onto a substrate by aerosol spraying. Aerosol spraying involves enclosing a liquid composition and a volatile propellant in a common container, then using the positive pressure generated by the propellant to drive the composition from the container. For example, the 3M brand Paint Defender Spray Film uses a polyurethane composition that can be sprayed onto an exterior surface of a vehicle and then dried to form a clear and durable protective coating. Aerosol products are attractive to endusers because they are easy to use, shelf-stable, do not require mixing, and can lay down very smooth and uniform films.